The present invention relates to an apparatus for printing a color superposition image, and particularly relates to a roller offset printing apparatus adapted to production of patterns such as liquid crystal color filter patterns, exposure mask patterns, liquid crystal shadow mask patterns, etc.
Referring to FIGS. 12 and 13, there is shown a conventional roller offset printing apparatus which has a roller transfer drum 101 covered with a rubber sheet and adapted to be rotated by a rotation motor 102, a linear stage 103, pinions 101a and 101b provided at opposite ends of the roller transfer drum 101, racks 103a and 103b provided at opposite side edges of the linear stage 103 so as to be engaged with the pinions 101a and 101b respectively, a master plate 106 and a work plate 107 disposed at a predetermined interval on the linear stage 103, and ink pits 121 (the diagrams (b) and (c) of FIG. 13) for reserving ink in a surface of the master plate 106.
As shown in the diagram (a) of FIG. 13, the pinions 101a and 101b are engaged with the racks 103a and 103b respectively, so that the roller transfer drum 101 moves while rotating on the linear stage 103 when the rotation motor 103 is driven. When the roller transfer drum 101 rotates on the surface of the master plate 106, ink 120 in the ink pits 121 is transferred onto the outer circumferential surface of the roller transfer drum 101 (the diagram (b) of FIG. 13). When the roller transfer drum 101 further rotates on the surface of the work plate 107, the ink 120 is transferred from the outer circumferential surface of the roller transfer drum 101 onto the surface of the work plate 107 (the diagram (c) of FIG. 13).
Thus, the conventional apparatus uses such a driving system that the roller transfer drum 101 is rotated through the engagement between the pinions 101a and 101b and the racks 103a and 103b as described above. Accordingly, a displacement may be produced between an original image on the master plate 106 and a printed image on the work plate 107 because of backlash, abrasion, etc. This arouses a problem in superposition printing.
Further, in the conventional apparatus, the roller transfer drum 101 is supported at its opposite ends by sliding bearings, tapering roller bearings, or the like. Accordingly, the rotational resistance of the roller transfer drum is large, so that there is a tendency of sliding of the roller transfer drum on the surface of the master plate 106 or on the surface of the work plate 107. There arises a problem in that transmission of movement cannot be made sufficiently.
Furthermore, in the conventional apparatus shown in FIG. 12, the distance between the master plate 106 and the work plate 107 is determined on the basis of the diameter of the roller transfer drum 101. That is, the distance is determined so that the rotation angular position of the roller transfer drum 101 in a first transfer start position where the roller transfer drum 101 is brought into contact with the surface of the master plate 106 so as to start to move while rotating on the surface of the master plate 106 in order to transfer the ink 120 in the ink pits (patterned recesses) in the surface of the master plate 106 onto the outer circumferential surface of the roller transfer drum 101 is made to coincide with the rotation angular position of the roller transfer drum 101 in second transfer start position where the roller transfer drum 101 starts to move while rotating on the surface of the work plate 107 in order to transfer the ink from the outer circumferential surface of the roller transfer drum 101 onto the surface of the work plate 107. Accordingly, when the diameter of the drum is changed, the rotation angular positions of the roller transfer drum 101 in the two transfer start positions are shifted from each other. As a result, there arises a problem in that the pattern (the ink 120 in the patterned recesses 121) in the surface of the master plate 106 is printed in a different position of the surface of the work plate 107. Accordingly, in the conventional apparatus, the printing start portion of the master plate 106 becomes different from the printing start position of the work plate 107 if the drum is changed in diameter.
If a position adjusting mechanism for changing the distance between the master plate 106 and the work plate 107 in accordance with the drum diameter is provided for the two plates, the drum diameter can be changed in the conventional apparatus. In this case, however, there arises a problem in that not only the size of the apparatus as a whole increases but the cost of production thereof increases.